End of 0 .25 .50 .75 Time in capillary (sec) •Fig. 24.3 Uptake of Nitrous Oxide (N2O), Carbon Monoxide (CO), and O2in Blood in Relation to Their Partial Pressures and the Transit Time of the Red Blood Cell in the Capillary. Forgasesthatareperfusionlimited(N2OandO2),theirpartialpressureshaveequilibratedwithalveolarpressurebeforeexitingthecapillary.Incontrast,thepartialpressureofCO,agasthatisdiffusionlimited,doesnotreachequilibriumwithalveolarpressure.Inrareconditions,O2uptakecanbecomediffusionlimited.
Diffusion limitation for O2 and CO2 would occur if red blood cells spent less than 0.25 seconds in the capillary bed. This is occasionally the case in very fit athletes during vigorous exercise and in healthy subjects who exercise at high altitude.
Oxygen is carried in blood in two forms: dissolved O2 and O2 bound to Hgb. The dissolved form is measured clinically in an arterial blood gas sample as the partial pressure of arterial oxygen (PaO2). Only a small percentage of O2 in blood is in the dissolved form, and its contribution to O2 transport under normal conditions is almost negligible. However, dissolved O2 can become a significant factor in conditions of severe hypoxemia. Binding of O2 to Hgb to form oxyhemoglobin within red blood cells is the primary transport mechanism of O2. Hgb not bound to O2 is referred to as deoxyhemoglobin or reduced Hgb. The O2-carrying capacity of blood is enhanced about 65 times by its ability to bind to Hgb.
Hgb is the major transport molecule for O2. The Hgb molecule is a protein with two major components: four nonprotein heme groups, each containing iron in the reduced ferric (Fe+++) form, which is the site of O2 binding, and a globin portion consisting of four polypeptide chains. Normal adults have two α-globin chains and two β-globin chains (HgbA), whereas children younger than 6 months of age have predominantly fetal Hgb (HgbF), which consists of two α chains and two γ chains. This difference in the structure of HgbF increases its affinity for O2 and aids in the transport of O2 across the placenta. In addition, HgbF is not inhibited by 2,3-diphosphoglycerate (2,3DPG), a product of glycolysis; thus O2 uptake is further enhanced.
Binding of O2 to Hgb alters the ability of Hgb to absorb light. This effect of O2 on Hgb is responsible for the change in color between oxygenated arterial blood (bright red) and deoxygenated venous blood (dark red-bluish). Binding and dissociation of O2 with Hgb occur in milliseconds, thus facilitating O2 transport because red blood cells spend only 0.75 seconds in the capillaries. There are approximately 280 million Hgb molecules per red blood cell, which provides an efficient mechanism to transport O2. Myoglobin, a protein similar in structure and function to Hgb, has only one subunit of the Hgb molecule. It aids in the transfer of O2 from blood to muscle cells and in the storage of O2, which is especially critical in O2-deprived conditions.